Dauragon shirtless:
Complete the game for a third time using any character. After defeating Dauragon for the second time, he will get up again and fight topless. Defeat him to unlock him in versus and survival modes.

Dauragon fighting with one arm:
Play the game in story mode and defeat Dauragon at his first encounter. The one armed fighting version of Dauragon will be unlocked in versus and survival modes.

Dominique:
Play the game in story mode until Dominique destroys the group of PD-4s to unlock her in versus and survival modes.

Kaldea:
Defeat Kaldea before the final encounter with Dauragon in story mode to unlock her in versus and survival modes.

Kou with MSF costume:
Play through the entire game, only selecting Kou. Save the game after Kou enters the Mikado building as a MSF soldier. The MSF costume for Kou will now be unlocked in versus and survival modes.

Leann Caldwell:
Play through the entire game, only selecting Kou. After you defeat Dauragon for the final time you will view a FMV sequence where you have to fight Leann. Defeat her to unlock her in versus and survival modes.

Master Wong:
Select Volt or Kou in their story line through the Mikado Building and have either of them fight Kaldea. Select Sion for the fight with Dauragon. After the battle, Sion remembers a battle with Master Wong. Defeat him to unlock Master Wong in versus or survival modes.

Mugetsu with mask:
Successfully complete the Mikado Building-Hanging Garden level to unlock Mugetsu with his mask in versus and survival modes.

Mugetsu without mask:
Successfully complete the Galeos-Mid Flight level to unlock Mugetsu without his mask in versus and survival modes.

PD-4:
Defeat the PD-4 during story mode to unlock it in versus and survival modes.

Sion with black hood:
Successfully complete survival mode by defeating the black-hooded version of Sion to unlock that costume in versus and survival modes.

Wong Leung:
Use only Volt or Kou through the entire game. However, when you are going to fight Threw for the last time, select Sion. When you finish the game, Sion will have a flashback. Defeat Wong Leung to unlock him in versus mode.

Hint - Defeating most Bosses easily:
Play as Kou. After you have gotten his uppercut attack, wail. If you timed correctly, you can "juggle" the enemy in the air several times.

Hint - Kou: Phone conversation:
Take Kou on the first battle against MSFs at Fate. When Kou calls Leann, you will hear what she says to Kou.